The most distinctive mark of substance appears to be that, while remaining numerically one and the same, it is capable of admitting contrary qualities. From among things other than substance, we should find ourselves unable to bring forward any which possessed this mark. Thus, one and the same colour cannot be white and black. Nor can the same one action be good and bad: this law holds good with everything that is not substance. But one and the selfsame substance, while retaining its identity, is yet capable of admitting contrary qualities.
This feature is effective is dividing substance from accident, but not in dividing substance from the parts of substance. Surfaces, flesh, the soul, and the mind can all admit contrary qualities as well. We could target just substance by saying that admits of contrary qualities while remaining one and the same whole.